I agree with John. In this case, we know the customer, have several accounts, and have seen their books. They are properly account for taxes on paystubs even though the bonus is given in cash.
On a personal note, I have worked for employers who gave cash bonuses. They included the full amount in the YTD pay, but did not withhold. Tax payments on the bonuses were the responsibility of the employee at tax time (basically the tax refund, if any, was reduced by any tax due on the bonus since the bonus was listed on the w-2).
I can't herd the cats anymore, so I just set up the electric fences and let them fry when they stray out of bounds.